Hydraulic fracturing, or fracing, is used to initiate/stimulate oil or gas production in low-permeability reservoirs. Hydraulic fracing has become particularly valuable in gas reservoirs wells and has been a key factor in unlocking the potential of unconventional gas plays, such as coal-bed methane, tight gas and shale gas reservoirs.
In hydraulic fracing, a fluid is injected into a well at such high pressures that the structure “cracks”, or fractures. Fracing is used both to open up fractures already present in the formation and to create new fractures. These fractures permit hydrocarbons and other fluids to flow more freely into or out of the well bore. Desirable properties of a hydraulic fracturing fluid may include high viscosity, low fluid loss, low friction during pumping into the well, stability under the conditions of use such as high temperature deep wells, and ease of removal from the fracture and well after the operation is completed.
Slick Water fracs have become more common, as they tend to be the least expensive of the fracture fluids. As part of the frac procedure, propping agents, or proppants, are often injected along with the fluid to “prop” open the new fractures and keep the cracks open when fracturing fluid is withdrawn. Hybrid fracs which are a combination of slick water and conventional frac technology are also becoming popular. A number of different proppants can be used such as sand grains, ceramics, sintered bauxite, glass or plastic beads, or other material. Thus, it is also important that the fracturing fluid be able to transport large amounts of proppant into the fracture.
Depending on the particular fracing operation, it may be necessary that the fluid be viscosified to help create the fracture in the reservoir and to carry the proppant into this fracture. In Hybrid fracs, crosslinkers could be added at the frac site, as the viscosity would be too high to pump through a pipeline. The high gel loading for non crosslinked Hybrid fracs would require that additional polymer be added at the frac site. Thus, water-based fracing fluids often include friction reducing polymers and/or viscosifiers such as polyacrylamides and polymethacrylamides, cross-linked polyacrylamides and cross-linked polymethacrylamides, polyacrylic acid and polymethacrylic acid, polyacrylates, polymers of N-substituted acrylamides, co-polymers of acrylamide with another ethylenically unsaturated monomer co-polymerizable therewith, 2-acrylamido-2-methylpropane sulfonic acid, polyvinyl pyrollidones, guar, substituted guars, other biopolymers such as xanthan such as xanthan gum, welan gum and diutan gum, derivatized biopolymers such as carboxymethyl cellulose, and other mixtures of polymers. Other chemicals such as scale inhibitor to prevent scaling, oxygen scavengers, H2S scavengers, biocides, and the like, may also be added.
It was common practice in the industry at one time to batch mix fracturing fluids at the well site. This was very costly and dependent upon water being present or being transported to remote sites and the bags of polymer, chemicals, etc. being transported on site. Further, incomplete mixing of the polymer and water was also a problem. If the dispersion of the polymer is incomplete, clumps of partially hydrated polymer can form, which clumps are commonly referred to in the industry as “fisheyes”.
More recently, liquid polymers, such as DynaFrac™ HT fluids, are being brought to the well site. However, the price of the premixed polymer itself and the costs to transport these large totes of liquid polymer make this a very costly alternative.
The present invention addresses these problems and provides a more cost effective process for preparing hydraulic fracturing fluid.